1. Technical Field
The present invention relates to a capacitance sensor, an acoustic sensor, and a microphone. More specifically, the present invention relates to a capacitance sensor configured by a capacitor structure including a vibration electrode plate (diaphragm) and a fixed electrode plate. The present invention also relates to an acoustic sensor (acoustic transducer) that converts acoustic vibration into an electric signal to output the electric signal and a microphone using the acoustic sensor. In particular, the present invention relates to a minute-sized capacitance sensor and a minute-sized acoustic sensor that are manufactured by using an MEMS (Micro Electro Mechanical System) technique.
2. Related Art
As a small-sized microphone mounted on a mobile phone or the like, an electret condenser microphone (Electret Condenser Microphone) has been popularly used. However, the electret condenser microphone is weak against heat, and is inferior to a MEMS microphone in corresponding to digitalization, reduction in size, high-functionalization/multi-functionalization, and power saving. For this reason, at present, the MEMS microphone has been popularized.
The MEMS microphone includes an acoustic sensor (acoustic transducer) that detects acoustic vibration and converts the acoustic vibration into an electric signal (detection signal), a drive circuit that applies a voltage to the acoustic sensor, and a signal processing circuit that performs signal processing such as amplification to the detection signal from the acoustic sensor to output the resultant signal to the outside. The acoustic sensor used in the MEMS microphone is an electrostatic capacitance acoustic sensor manufactured by using the MEMS technique. The drive circuit and the signal processing circuit are integrally manufactured as an ASIC (Application Specific Integrated Circuit) by using a semiconductor manufacturing technique.
In recent years, a microphone is required to high-sensitively detect sound having a low sound pressure to a high sound pressure. In general, the maximum input sound pressure of a microphone is limited by a total harmonic distortion (Total Harmonic Distortion). This is because, when sound having a high sound pressure is to be detected by a microphone, harmonic distortion occurs in an output signal to damage sound quality and accuracy. Thus, when the total harmonic distortion can be reduced to a low level, the maximum input sound pressure is increased to make it possible to widen a detection sound pressure range (to be referred to as a dynamic range) of the microphone.
However, in a general microphone, trade-off between improvement of detection sensitivity of acoustic vibration and a reduction in total harmonic distortion is established. For this reason, in a high-sensitive microphone that can detect sound having a small volume (low sound pressure), a total harmonic distortion of an output signal increases when the microphone receives sound having a large volume, and, therefore, the maximum detection sound pressure is limited. This is because the high-sensitive microphone outputs a greater signal and easily causes harmonic distortion. In contrast to this, when the harmonic distortion of the output signal is reduced to increase the maximum detection sound pressure, the sensitivity of the microphone is deteriorated to make it difficult to detect sound having a small volume with high quality. As a result, in a general microphone it is difficult to have a wide dynamic range from a small sound volume (low sound pressure) to a large sound volume (high sound pressure).
In the technical background, as a method of achieving a microphone having a wide dynamic range, a microphone using a plurality of acoustic sensors having different detection sensitivities is examined. As such a microphone, for example, a microphone disclosed in Patent Documents 1 to 4 is known.
Patent Documents 1 and 2 disclose a microphone in which a plurality of acoustic sensors are arranged and a plurality of signals from the plurality of acoustic sensors are switched or merged with each other depending on sound pressures. In the microphone, for example, a high-sensitive acoustic sensor that can detect a sound pressure level (SPL) of about 30 dB to 115 dB and a low-sensitive acoustic sensor that can detect a sound pressure level of about 60 dB to 140 dB are switched and used to make it possible to configure a microphone that can detect a sound pressure level of about 30 dB to 140 dB. Patent Documents 3 and 4 disclose one chip on which a plurality of independent acoustic sensors are formed.
FIG. 1A shows a relationship between a total harmonic distortion and a sound pressure in a high-sensitive acoustic sensor in Patent Document 1. FIG. 1B shows a relationship between a total harmonic distortion and a sound pressure in a low-sensitive acoustic sensor in Patent Document 1. FIG. 2 shows relationships between average displacement amounts and sound pressures of diaphragms in the high-sensitive acoustic sensor and the low-sensitive acoustic sensor in Patent Document 1. When it is assumed that an allowable total harmonic distortion is 20%, the maximum detection sound pressure of the high-sensitive acoustic sensor is about 115 dB. In the high-sensitive acoustic sensor, since an S/N ratio is deteriorated when the sound pressure is smaller than about 30 dB, the minimum detection sound pressure is about 30 dB. Thus, the dynamic range of the high-sensitive acoustic sensor is, as shown in FIG. 1A, about 30 dB to 115 dB. Similarly, when it is assumed that an allowable total harmonic distortion is 20%, the maximum detection sound pressure of the low-sensitive acoustic sensor is about 140 dB. The low-sensitive acoustic sensor has a diaphragm area smaller than that of the high-sensitive acoustic sensor, and, as shown in FIG. 2, has an average displacement amount of the diaphragm smaller than that of the high-sensitive acoustic sensor. Thus, the minimum detection sound pressure of the low-sensitive acoustic sensor is larger than the high-sensitive acoustic sensor, i.e., about 60 dB. As a result, the dynamic range of the low-sensitive acoustic sensor is, as shown in FIG. 1B, about 60 dB to 140 dB. When the high-sensitive acoustic sensor and the low-sensitive acoustic sensor as described above are combined with each other, a detectable sound pressure range, as shown in FIG. 1C, becomes wide, i.e., about 30 dB to 140 dB.
Patent Document 1: Publication of US patent application No. 2009/0316916
Patent Document 2: Publication of US patent application No. 2010/0183167
Patent Document 3: Japanese Unexamined Patent Publication No. 2008-245267
Patent document 4: Publication of US patent application No. 2007/0047746